1) Field of the Invention
This invention relates to the control of the expansive properties of foaming or foamable adhesives (also referred to as back foam) in cellular core splice applications. This control is accomplished by partially or fully encapsulating the foaming adhesive with a supported film adhesive. This encapsulated adhesive arrangement is then put into the bond assembly between the honeycomb core units to be spliced together. The greatest benefits will be achieved when used in acoustic noise reduction panels where one face skin of the panel is perforated with small holes (for example, the diameter of the small holes may be 0.050 inch) to allow sound waves to enter. For instance, such cellular core splices occur in acoustic noise reduction panels used in airplanes in jet engine nacelle inlets, engine strut panels, thrust reverser blocker doors, and other types of panels where at least one face skin of the panel is perforated with small holes and where it is desirable to minimize the number of holes that get filled in with adhesive and thus become of no value in noise reduction. One goal of this invention is to improve jet airplanes and jet engines in the areas of noise reduction (quiet operation), appearance, and reliability.
2) Description of the Background Art
Foaming adhesives are used on internal structural bond applications where surfaces of metallic or nonmetallic cellular core, such as aluminum honeycomb core or composite honeycomb core, must be bonded together. Due to the ragged and erratic edges inherent in cellular core, adhesives with expansive properties are used to ensure adequate core cell wall coverage. Other adhesives do not expand to cover enough surface area of the erratic core edges. Therefore, an adhesive that will generate its own expansive pressure in the autoclave bonding process is used to accomplish good area coverage of the cell walls with adhesive.
The unique characteristics of foaming adhesives provide beneficial structural bond manufacturing capabilities and widespread applications. However, these manufacturing applications are not without serious consequences. A constant manufacturing concern is keeping foaming adhesives from expanding into places where they are not needed or desired. Such was the case in the development of a new noise reduction jet engine nacelle inlet, where improved appearance, noise reduction, and weight reduction were mandatory. The present invention was developed to enhance the newly designed nacelle inlet by replacing the prior art method of splicing honeycomb core material shown in FIG. 1.
FIG. 1 illustrates a prior art acoustic panel 10 constructed of an nonperforated inner skin 14, a perforated outer skin 16 having multiple small holes 18, two sections of honeycomb core material 12, 12a that are separated by gap 22 and which are adhesively bonded or spliced together by an environmentally durable foaming adhesive 20 that was foamed (by heating) and which thereby expanded and bonded to the walls 24, 24a of the honeycomb cells around the gap 22 that extends transversely across panel 10 in a direction that is perpendicular to the ribbon direction of the honeycomb core material 12, 12a. The cells around gap 22 will hereafter be referred to as the gap cells. Nonperforated inner skin 14 has been adhesively bonded to honeycomb sections 12, 12a by a supported film adhesive (described below) (not shown in FIG. 1). Perforated outer skin 16 has been adhesively bonded to honeycomb sections 12, 12a by an unsupported film adhesive (described below) (not shown in FIG. 1).
FIG. 1 also illustrates that some of foaming adhesive 20 bleeds out vertically thereby filling a large number of holes 28, 28a (which is not desirable) and that it also bleeds out horizontally at 26, 26a (which is also not desirable), thereby partially filling honeycomb cells adjacent the gap cells. Blocked holes 28, 28a in perforated outer skin 16, and covered acoustic septum (in the cells adjacent the gap cells), render both inactive for noise reducing acoustic purposes. For the sake of clarity, the acoustic septum in the cells of the core have been omitted from the present drawings, but it should be understood that acoustic septum are located in the cells of the cellular core. Cellular core with internal acoustic septum are described and shown in Harp et al. U.S. Pat. No. 4,265,955 and Diepenbrock, Jr. et al. U.S. Pat. No. 4,257,998 (both assigned to The Boeing Company, the assignee of the present invention). The splices in prior art acoustic panel 10 made by using uncontained foaming adhesive 20 are often 0.50 inch to 0.75 inch wide and have irregular and ragged patterns of filled holes 28, 28a.
As compared to the prior art acoustic panel 10 (FIG. 1), the acoustic panel 30 (FIG. 3) of the invention has a smaller number of filled holes 36, 36a and a smaller splice width measuring 0.10 inch to 0.15 inch wide. It also has a neater appearance and a more uniform pattern of filled holes 36, 36a.